Power converters are used to convert electrical energy from one form to another form. For example, direct current-to-direct current (DC-to-DC) power converters are used to convert power from one DC form to another DC form. Efficiency is often an important characteristic of power converters. Power converters with higher efficiencies dissipate less energy during the conversion process than power converters with lower efficiencies. This can have significant impact on, for example, the length of time that a device can be used before recharging of a power supply in the device is needed.
Switched mode power converters repeatedly turn various transistors on and off to control the conversion of power from one form to another. Phase shift modulation (PSM) is one type of control technique that can be used to control the transistors in a switched mode power converter. PSM control can maintain low switching losses and low noise by operating in a “zero-voltage switching” (ZVS) mode of operation. In this mode, transistors are switched on and off at or near a zero voltage, which can help to reduce or minimize switching losses in the transistors. Combined with high switching frequencies, ZVS can be used to achieve compact and low-loss power converters.
Unfortunately, there are various problems associated with conventional power converters that operate using zero-voltage switching. For example, losses within conventional power converters typically increase when lighter loads are connected to the power converters. Also, conventional power converters can experience excessive reactive power at higher input levels. One method of providing ZVS introduces a source of additional inductive (lagging) current into switching nodes of a PSM-controlled power converter. This approach helps to increase the power converter's efficiency at lighter loads, but it also typically compromises the power converter's efficiency at mid-range loads and full loads. This approach also does not address the problem of reducing reactive power processed by a power converter at higher input levels.